RUNAWAY STABILIZER
Condition: Uncommanded stabilizer trim movement occurs continuously.
Control column Hold firmly
Autopilot (if engaged) Disengage
Do not re-engage the autopilot.
Control airplane pitch attitude manually with control column and main electric trim as needed.Autothrottle (if engaged) Disengage
Do not re-engage the autothrottle.If the runaway stops after the auto pilot is disengaged.
CompleteIf the runaway continues after the autopilot is disengaged:
STAB TRIM CUTOUT switches (both) CUTOUT
If the runaway continues:
Stabilizer trim wheel Grasp and holdStabilizer Trim manually
Anticipate trim requirements.
The difference between an electric runaway and aerodynamic runaway can easily be recognized. In case of an electric runaway, the stabilizer trim runs at high speed immediately and the runaway stops when placing the stab cutout switches to CUTOUT.
Moving the control column opposite to the uncommanded trim motion opens the control column stabilizer trim cutout switches. In case of an electric runaway, this action stops the runaway. However, in case of an aerodynamic runaway, this action aggravates the situation. This may confuse the crew and complicates aircraft controllability.
RUNAWAYS STOPS when placing both switches to CUTOUT
If the autopilot was engaged :
- Main electric motor runaway commanded by the A/P
- Do not engage autopilot A and/or B
- Electric trim remains available through the control wheel trim switch
If the autopilot was not engaged :
- Electric motor runaway commanded by the control wheel trim switches
- Recycle / Verify trim switches on both control wheels
- Electric trim remains available through the A/P
RUNAWAY CONTINUES when placing both switches to CUTOUT
- Suspect an aerodynamic runaway ( dual failure of brake system)
- Do not displace the control column in the opposite direction. Use electric trim to counteract the trim runaway!
- Notice that the NTSB recommends a landing at the nearest suitable airport for a stabilizer aerodynamic runaway - this may also be a company SOP
- Never GRASP and HOLD the trim wheel as shown on the first illustration! Injury to fingers may occur when the handle passes by! Place the bottom of your hands in one go on top of the spinning trim wheel on the other pilot's side! If you do this deliberately and firmly, it will not burn your skin. I have done it a dozen times.
The horizontal stabilizer trim mechanism consists of a single Main Electric Trim Motor. The stabilizer is held in position by a dual brake system. Either stabilizer brake can maintain the stabilizer position. In the event of a dual brake failure, air load can drive the stabilizer to its mechanical stops (aerodynamic runaway).